Polyvinyl alcohol (hereinafter polyvinyl alcohol may be abbreviated to “PVA”) is known as a water-soluble synthetic polymer and used for various uses including a raw material for vinylon fiber, a coating and sizing agent for paper, a sizing agent for fibers, an adhesive for paper, a dispersion stabilizer for emulsion polymerization and suspension polymerization, a binder for inorganic substance such as a ceramic, a water soluble film, an optical film such as a polarizing film, and a medical material. PVA is also known to be eco-friendly and harmless to a living body.
In order to further expand the uses of PVA, attempts to modify PVA have been made. One example of such modification is introduction of a reactive carbonyl group into PVA, and in particular a polyvinyl alcohol resin having diacetone acrylamide introduced thereinto is known (hereinafter, polyvinyl alcohol resin may be abbreviated to “PVA resin”).
PVA having diacetone acrylamide introduced thereinto (hereinafter, PVA having diacetone acrylamide introduced thereinto may be abbreviated to “DA-PVA resin”) is used as a film easily soluble in cold water or a protection material for a light-sensitive flat printing plate because DA-PVA resin hardly suffers from decrease in water solubility resulting from hydrogen bonding between the hydroxyl groups. DA-PVA resin is also used as a protection layer for a thermal recording material, an ink-jet recording material, and a film, as well as an aqueous gel because DA-PVA resin can form a highly water resistant material through reaction of the reactive carbonyl groups present in the resin with a carbonyl-reactive crosslinking agent (Patent literature 1 to 8).
DA-PVA resin is thus used as the materials described above but sometimes is not sufficiently transparent in the form of an aqueous solution due to the reactive carbonyl groups contained therein, and has problems of, for example, unfavorable appearance resulting from decreased transparency in the form of a film or an aqueous gel, as well as of unfavorable effects on ink acceptability or color appearance, insufficient water resistance, etc.